1. Field of the Invention
This invention relates to a method of producing photomasks and, more particularly, it relates to a method of producing photomasks having improved durability.
2. Description of the Prior Art
Hitherto, emulsion masks each having a desired pattern at the emulsion layer coated on a transparent glass substrate have been widely used as photomasks used for making semiconductor devices such as IC's, LSI's, etc., printed circuit plates, shadow masks, etc. Also, in selectively exposing a layer of a photoresist on a substrate to be treated such as a semiconductor wafer using an emulsion mask, a so-called contact exposure method in which, the photoresist layer on the wafer is exposed in a close contact relationship with the emulsion mask having a mask pattern therein is most generally employed.
However, since in the close exposure method using an emulsion mask as described above the emulsion mask having the mask pattern is closely contacted with the photoresist layer, the comparatively soft emulsion mask having the mask pattern therein tends to be damaged. This results in difficulties in using the method described above because the number of times the emulsion mask is usable is decreased, the masks must be changed frequently and large numbers of masks are needed, much time is required for exchanging the masks, and further it becomes difficult to automate the mask alignment.
One method for preventing emulsion masks having mask patterns therein from being damaged is a method in which a layer of a metal or an oxide thereof such as chromium or chromium oxide is used to form a mask pattern in place of the emulsion layer on a glass substrate. However, in this method, the durability is limited since in this method although damage of the mask pattern may be minimized the pattern is formed on a glass support as a convex pattern, and further when, for example, a silicone wafer is contact-printed using a mask having such a convex pattern, the convex structure portion is pressed against the surface of the wafer. Thus, both photoresists and photomasks tend to be damaged and, in particular, such is severe for photomasks. Also, in conventional photomasks where many are consumed, the use of expendable substrates is disadvantageous and hence it is necessary to use inexpensive substrates. Therefore, if the size of the photomask substrate is increased, the flattness of the photomask is reduced, which results in limitations on the formation of fine patterns.
Various attempts have hitherto been made for overcoming these difficulties inherent in conventional photomasks. One of these methods is described in U.S. Pat. No. 3,561,963 and also in U.S. Pat. No. 3,732,792, column 9, lines 17-37. According to these conventional methods, metal ions are diffused imagewise into a glass support to form portions, which obstruct the transmission of ultraviolet light in the support, and the colored portions in the glass support are used as a mask image. Since in this method, a mask image is formed in a glass support and hence the mask image formed is flat without any convex portions being formed, the difficulties described above are all overcome and a photomask having a very excellent durability can be obtained.
The most advantageous method for obtaining such a photomask is disclosed in U.S. Pat. No. 3,561,963. This method is carried out by forming uniformly a photoresist layer on a glass support, imagewise exposing the photoresist layer followed by development to form a relief image and, at the same time, uncover the surface of the glass support at the non-relief image-bearing portions, applying uniformly a metal ion-supplying material over the entire surface of the relief image-bearing glass support, and then heating the glass support to diffuse the metal ions into the surface of the glass support at the uncovered portions, whereby the uncovered portions are selectively colored to provide a mask image.
However, it has now been found in such a method, after applying the ion-supplying material onto the surface of a relief image-bearing glass support, when the glass support was heated with the relief image remaining on the support for coloring the uncovered portions only of the glass support, the surface of the glass support was colored not only at the uncovered portions but also at the relief image-bearing portions, that is, the photoresist layer used in U.S. Pat. No. 3,561,963 described above was not useful as a material for preventing the ions from the ion supplying material from diffusing into other portions of the glass support surface than the uncovered portions.
That is, when, after applying onto a glass support either a negative-type/photoresist, KTFR (tradename, produced by Eastman Kodak Co.) or a positive-type photoresist, AZ-1350 (tradename, produced by Shipley Co.), which are the most popular photoresists used in this field, at a thickness of about 1 to 3 microns, a resist pattern was formed in a conventional manner and then a silver ion-supplying material was applied thereon followed by a heat treatment, the silver ions diffused into the overall surface of the glass support, whereby the resist pattern-bearing portions and the non-resist pattern-bearing portions were almost similarly colored in using AZ-1350, while the coloring of the glass support at the resist pattern-bearing portions was slightly less than that at the non-resist pattern-bearing portions in using KTFR but the contrast between the two portions was very low even in the latter case.
Therefore, in the process described in U.S. Pat. No. 3,561,963, a method is employed in which an ion-supplying material layer such as a deposited layer of copper is uniformly formed on the surface of a glass plate having formed thereon a relief image of a photoresist, then the relief image is removed from the glass support together with the ion-supplying material layer formed on the relief image having the ion-supplying material layer on the non-relief image-bearing portions, i.e., formed directly on the glass surface, and the glass support was heated to imagewise color the surface of the glass support.
However, in such a method, the steps are complicated since additionally, after forming an ion-supplying material layer, the relief image and the ion-supplying material layer formed thereon must be selectively removed leaving the ion-supplying material layer on the non-relief image portions only, and the removal of the relief image must be performed by the action of a strong solvent such as acetone. Hence it is difficult to remove selectively the relief image-bearing portions only and disadvantageously there is the possibility that the ion-supplying material layer at the relief image-bearing portions is partially removed with the removal of the relief image to spoil the mask image formed. Furthermore, the most important fault in this method is because the ion-supplying material must be selectively removed in accordance with the relief image present on the support and hence the material used must be capable of forming a continuous coating such as a deposited layer, which results in limiting usable ion-supplying materials to a very narrow range and also makes it difficult to use ion-supplying materials having a large ion-supplying effect as will be explained below.